It's likely to early to say as many have suggested, but if I HAD to pick one now I'd say Coliacovo.
I caught 2 Giants home games while I was home over Christmas and was not overly impressed with Meszaros. For sure he has evident skills, most noticeably in his puck-poise and passing, but defensively he was not impressive. He also skates well for sure, but Coliacovo is a better skater AT THIS TIME and that's LIKELY to stay the same as Cola is a top-notch skater. Granted, though, Meszaros is younger and is adapting
Conversely, Cola has become very good defensively and has bulked up to allow him to be more physical, which he was very effectively in St. Johns this year. Add to that he has good offensive ability, projects to be a future PP qb and has good point shot and I've got to give the nod to Coliacovo at this time.
IMO their potential as defenceman is roughly equal, but I think Coliacovo will be better